MacOS – Why is the Mac’s CPU throttled? (Turbo Boost Ratio)

cpumacbook promacosturbo-boost

AnandTech has a instructions1 on how you can check the Turbo Ratios for the Mac's CPU. Since they are logged, the information is very easy to find.

In Mountain Lion you can do this the following way:

  1. Open /Applications/Utilities/Console.app
  2. Open the system.log on the left side.
  3. Search for entries containing Turbo Ratios

What you'll find is something like this:

Dec 16 20:11:47 localhost kernel[0]: AppleIntelCPUPowerManagement: Turbo Ratios 0035

And this means:

The line above lists turbo ratios as 0035. The four digit number is the turbo ratio for the active cores
(4C/3C/2C/1C). Depending on the active cores, the clock speed
can vary. Each number is hexadecimal.

The first two numbers are 0s because the CPU doesn't have more than
two cores and thus doesn't support any turbo ratios when 4 or 3 cores
are active (2012 Macbook Air, i5-3427U). The third number tells us the
maximum turbo boost with two cores active: 3. That's 3 bins, where
each bin is 100MHz, or 300MHz above the stock 1.8GHz operating
frequency (2.1GHz).

The fourth number gives us max turbo when only a single core is
active: 5. Five bins is 500MHz, which on top of the 1.8GHz base
frequency gives us 2.3GHz.

(Edited text from AnandTech to fit my CPU.)

However, according to notebookcheck.net2 the Turbo Ratio should be 008A or 1.8GHz/2.6GHz/2.8GHz. This would be in line with Apple's advertisement3 for the 2012 13" Macbook Air:

enter image description here

This raises the obvious question: Why is my Mac's CPU throttled?

  • I wish I were able to use the MSR tools to get more information about the clock speeds, but for them to work I need to boot in 32-bit kernel mode which is not available in Mountain Lion anymore.

  • MacCPUID4 is an official tool by Intel which you can use to calculate the current CPU clock. The current version 2.1 has been released on the 10/31/2012 but still seems buggy. No matter how I stress my CPU, it always calculates that the clock is at a constant 2.3 GHz.

Best Answer

So, I ran some Cinebench R11.5 benchmarks to get more information about the single threaded performance of my i5-3427U1 (1.8-2.8 GHz) CPU:

1 thread → 1.10

I also contacted the author of the linked article by AnandTech - @anandshimpi2. Anand Lal Shimpi was so kind to send me a benchark chart which compared the single threaded performance of the 2011 13" Macbook Airs with the 2012 13" Macbook Airs using Cinebench R11.5.

Into this picture, I have added information about the maximum single threaded CPU clocks along with a comparison of the measured performance such as Anand Lal Shimpi has suggested.

enter image description here
[image source]

In this info graphic I am comparing specifically:

  • 2011 13" Macbook Air, i5-2557M 1.7GHz/2.4GHz/2.7GHz
  • 2012 13" Macbook Air, i5-3427U 1.8GHz/2.6GHz/2.8GHz

As you can see, the single threaded clock of the 2012 model is 3.7% increased in turbo boost mode.

However, the benchmark score saw a 9.9% increase which is not simply due to the higher clock, but also due to the more efficient Ivy Bridge architecture which is about 5-15% faster3 than Sandy Bridge.

In sum, the benchmark scores are as expected whereas the Turbo Ratios log entries are not. Anand Lal Shimpi had "figured it was a bug, submitted it to Apple back in July"4.

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